Yield stress and onset of nonlinear time-dependent rheological behaviour of gellan fluid gels

We present a method to determine simultaneously the yield stress and the onset of non-linear time-dependent rheological behaviour applied to gellan gum fluid gels (0.2 wt% gellan gum, 0.2 wt% Ca+2 as promoters of the formation the gel). It consists of creep-recovery-creep tests for shear stresses in the range 2-6 Pa, avoiding the effect of shear history. Results obtained at the lower stresses applied showed a response dominated by viscoelasticity. At a given higher stress the variability of results obtained allowed the yield stress to be determined, which is associated to the occurrence of shear banding. Above this practical yield stress the results showed a clear thixotropic response. A flow curve free of shear history effects could be derived from series of creep experiments and showed very shear thinning behaviour. The shear rate dependence of viscosity within the fully developed shear flow region fitted the power law equation with a flow index of 0.2 and the comparison of viscosities calculated at different shear times illustrated the thixotropic behaviour of gellan fluid gels.